Addressing the issue motivates researchers to design reconfigurable intelligent surfaces (RISs) to create LoS links by smart reflection and to direct them in desired directions [1]}, [2]}. RIS is an artificial metasurface composed of passive reflecting elements that can adjust the amplitude and phase of a reflected signal [3]}, [4]}. Moreover, since the array architecture of the RIS is passive, it features low power consumption [5]}. Therefore, RISs are expected to be deployed in future wireless networks to enhance their performance. In [6]}, [7]}, the authors improved the energy-efficiency by jointly allocating transmit power and adjusting phase shifts of RIS. In [8]}, [9]}, [10]}, the researchers proposed various RIS transmission strategies, including a combined design of continuous transmit precoder and discrete phase shifts [8]}, single-to-multi-beam training methods by partitioning RIS array and designing their beam directions [9]}, and constructing multiple-input-multiple-output quadrature amplitude modulation (MIMO-QAM) under the RIS hardware constraints such as discrete phase shift and phase-dependent amplitude [10]}. Moreover, maximizing the spectral efficiency is studied in [14]}, [15]}, [16]}. The prior works, however, mainly focuses on deploying RIS in a terrestrial environment that includes buildings and walls. Such an environment poses several limitations on communication performance. First of all, in urban areas with many buildings, reliable communication requires multiple reflections, which requires the installing of many RISs to avoid severe signal attenuation [17]}. Moreover, the terrestrial RIS can only reflect the signal from the source to the destination on the same side of the RIS, which cannot achieve isotropic reflection with \(0^{\circ }\sim 360^{\circ }\)  arrival-angles. Although some attempts have been made to implement the RIS on the aerial platforms [18]}, [19]}, [20]}, only the 2D coverage is considered or the fixed transmit power assumed, without considering the energy-efficiency problem.
